1. Field of the Invention
The present invention relates to a radiography apparatus for performing tomosynthesis radiography to generate slice images of a subject.
2. Description of the Related Art
In recent years, tomosynthesis radiography was proposed to observe affected regions of patients in more detail by a radiography apparatus using X-rays. In tomosynthesis radiography, radiography is performed by moving an X-ray tube to different positions, and by irradiating a subject by the X-ray tube at different angles from the different positions. Further, images obtained by radiography are added to obtain an image in which a desired cross section of the subject is emphasized. In tomosynthesis radiography, the X-ray tube is moved parallel to an X-ray detector, or in such a manner to draw a circle or an ellipse or oval, based on the characteristic of a radiography apparatus and the kind of slice images (tomograms) to be obtained. The subject is radiographed at different radiation angles to obtain a plurality of radiographic images, and a slice image of the subject is reconstructed from the plurality of radiographic images.
In tomosynthesis radiography as described above, a method for obtaining slice images having different slice thicknesses has been proposed. In the method, a plurality of radiographic images are combined by weighted addition and the like to obtain the slice images (please refer to U.S. Pat. No. 7,515,682 (Patent Document 1)). Further, a method for preventing inconsistency at the edges of images and generation of a ghost at a position in which a subject should not be present has been proposed. In the method, a plurality of radiographic images are processed by non-uniform weighted addition (please refer to U.S. Pat. No. 6,980,624 (Patent Document 2)). Further, a method using order statics, such as a minimum value, a maximum value and a median, in addition to an average value has been proposed to reconstruct an image from radiographic images (please refer to U.S. Pat. No. 6,707,878 (Patent Document 3)).
In tomosynthesis radiography, it is known that the slice thickness of a generated slice image, in other words, the resolution of the slice image in the depth direction of a subject (the direction in which radiation propagates) depends on a slice angle adopted at the time of radiography (please refer to T. Shiomi, “The principle and clinical application of Tomosynthesis”, Journal of Japanese Society of Medical Imaging and Information Sciences, vol. 24, No. 2, pp. 22-27, 2007 (Non-Patent Document 1)). The slice angle refers to an angle formed, at a base point on a base plane (base surface), by lines connecting the base point and two end points that define the movement range of an X-ray tube. The base plane defines the range for obtaining slice images. For example, the base plane is a surface of an X-ray detector, a surface of the top board of a radiography table on which a subject (patient) is placed, a surface of a region of interest, the surface closest to the X-ray detector, when the region of interest has been set, an arbitrary cross section of the subject, or the like. According to Non-Patent Document 1, in radiographic images, the divisibility of an object in a subject in the thickness direction of the subject becomes higher as the slice angle is larger. Therefore, the slice thickness of a slice image becomes thinner. In contrast, in radiographic images, the divisibility of an object in a subject in the thickness direction of the subject becomes lower as the slice angle is smaller. Therefore, the slice thickness of the slice image becomes thicker.
In tomosynthesis radiography, X-rays that are output from each radiation source position to a subject disperse in cone form. Therefore, if a position in the depth direction of the subject differs, the slice angle differs. Therefore, if all of radiographic images obtained by radiography are used to generate a slice image of each slice plane, the slice angle differs depending on the position in the depth direction of the slice plane. Consequently, the slice thickness of each of the slice images generated on respective slice planes differs from each other.